Add XML documentation for Android.Animation

The `Android.Animation` namespace contains **253** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Animation/**' --glob 'ns-Android.Animation.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **229** to **253** (increased by **24**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start with the Android.Animation documentation files under docs\xml, using the provided rg command to locate all 253 “To be added” placeholders, and review related documentation PR #644 for context. Replace the placeholders with accurate reference text and rerun the scan; done means no remaining placeholders in this namespace, excluding docs\xml\index.xml.

Written by the indexing model from the issue text.